Beside fungus resistance, dimensional stability in the case of changing atmospheric moisture content, a smooth surface and workability are the most important characteristics required; for in the case of dimensional changes, e.g. the sealing and heat insulation function of windows and doors, is no longer guaranteed and in the case of a surface high in stresses and having cracks, the wood is not only more susceptible to maintenance but neither can it be coated with plastics in a satisfactory quality. A smooth surface after coating presupposes a smooth, crackless surface of the material used in the case of modern coating processes, e.g., in extrusion coating techniques. In a plastic coating too, dimensional stability and fungus resistance must be guaranteed in order not to have to fear a destruction of the wood and subsequent separation of the plastic coating in the case of any damage to the coating.
From the German Pat. No. 2 263 758, it is known that as a result of the thermal treatment of wood in the case of a wood moisture content between 15 and 30% and temperatures between 100.degree. and 180.degree. C., a decrease of the swelling may be achieved. In the case of thick pieces of wood however, a formation of cracks in the wood will occur easily in the case of such moisture contents during the thermal treatment, which formation of cracks is the more distinct, the higher the wood moisture and the higher the reaction temperatures. The causes lie in the stresses which develop according to experience in the case of too rapid drying. At the same time beechwood is considerably more sensitive than fir or red firwood; on the other hand, beechwood may experience a considerable increase in value by a crackfree stabilization of its dimension. Thin pieces of wood, such as for example veneers with initial moisture content above 10% will not suffer any formation of cracks during the treatment but they emerge highly corrugated from the treatment. This leads to difficulties in the case of subsequent gluing, for a uniform machine application of the glue is hardly possible then and in the case of the intensities of pressure necessary for the gluing, the formation of a crack occurs frequently. A further disadvantage lies in the long reaction times at temperatures below 180.degree. C., as a result of which the profitability of the process is impaired.
From the German OS No. 2 654 958, it is known furthermore to modify wood by a multi-step, elaborate process in aqueous solution with the addition of surface active substances and alkalies at pressures up to 3 bar and temperatures up to 130.degree. C. In that case an increase of the strength of the wood is achieved and in addition the wood becomes more fungus resistant and more uniform in its coloration. However, in that case, we are dealing with an elaborate and expensive process, especially with a view to the reconditioning of the aqueous solution for reasons of protection of the environment.